It seems established that the cell surface is implicated in the expression of the neoplastic state. Lectins (cell agglutinins) of various types are proving useful in delineating the changes on the surface of mammalian cells following viral transformation. Some of the properties of the transformed-cell surface can be operationally reproduced by treating normal cell surfaces with trypsin. Currently it is felt that the increased agglutinability of transformed or trypsin-treated cells in the presence of lectins can be explained by a redistribution of cell receptors without any increase in the number of receptors. One major aim is to re-examine this explanation by quantitatively measuring the interaction of lectins with receptors on human erythrocytes treated with trypsin. The use of altered erythrocytes as a means to immobilize receptor sites is proposed to supplement this information. Published studies demonstrating increased lectin-mediated agglutinability have exclusively utilized experimentally transformed cells. Human neoplastic cells have not been so used; the extent of lectin-mediated agglutinability of human neoplastic cells needs to be demonstrated. Leukemic patients can provide naturally dissociated cells for binding studies outlined above (non-leukemic serving as controls). In addition, erythrocytes from leukemic patients are of great interest in view of reported cases of blood group alterations. Future applications to solid human tumors are discussed.